Finnish Patents 60 825, 74 654 and 77 405 disclose manufacturing lines for ribbed pipes, wherein a formable mass brought into a plastic state is extruded by means of a die between movable forming means pressed against one another to achieve continuous production of ribbed pipe. With these prior art solutions, ribbed pipe products having a good quality and being commercially successful are achieved.
In prior manufacturing lines for ribbed pipes, cooling of the pipe has been arranged to be two-sided, i.e. both the forming means disposed against the exterior surface of the pipe and the interior mandrel supporting the pipe through the corrugator are cooled. However, slight waviness has been found on the inner surfaces of ribbed pipes thus produced, despite the mandrel arrangement suggested by Finnish Patent No. 74 654. It has been found that this is caused by depressions at the rib flanges, resulting from the fact that the rib flanges are cooled at different rates at different points, i.e. faster at the rib flange surfaces pressed into the grooves of the forming means than at the foot of each rib flange. Uneven cooling will result in uneven shrinkage of the material, which is manifested as said waviness on the inner surfaces of the pipe.